1. Field of the Invention
The present invention relates to a projection exposure apparatus used in photolithography process for making semiconductor devices and liquid crystal display devices and, in particular, to a projection exposure apparatus equipped with an alignment system which captures images of a reticle alignment mark formed in a reticle and a substrate alignment mark formed in a photosensitive substrate, subjects thus captured images to image processing, and positions the reticle and the photosensitive substrate with respect to each other.
2. Related Background Art
In a photolithography process for making semiconductor devices, liquid-crystal display devices, CCD imaging devices, and the like, a projection exposure apparatus is used so as to expose and transfer a pattern image formed on a reticle or photomask (hereinafter simply referred to as reticle) onto a photosensitive substrate, such as semiconductor wafer or glass plate, which is coated with a photosensitive material. Since a number of layers of circuit patterns are formed on a photosensitive substrate in general, in order to improve the yield of semiconductor devices or the like manufactured, it is necessary for a high overlay accuracy to be maintained between the pattern that has already been formed in each shot area on the photosensitive substrate and a pattern image of the reticle to be transferred and exposed thereon. To this end, the position of a substrate alignment mark (hereinafter simply referred to as wafer mark) disposed at each shot area on the photosensitive substrate is detected with a high accuracy by means of an alignment system and, based on thus detected position, the positions of photosensitive substrate and reticle are spatially adjusted with respect to each other.
Such an alignment system is disclosed in U.S. Pat. No. 5,151,750 or the like and encompasses LSA (Laser Step Alignment) system in which an alignment mark formed like a dot series is irradiated with laser light, and light diffracted or scattered by the alignment mark is used to detect the position of alignment mark; LIA (Laser Interferometric Alignment) system in which a grating-like alignment mark is irradiated, for example, with laser beams whose frequencies are slightly different from each other respectively from two directions, the resulting two diffracted light components are caused to interfere with each other, and the position of alignment mark is measured from their phases; FIA (Field Image Alignment) system in which image data of an alignment mark captured under illumination of light having a broad wavelength bandwidth whose light source is a halogen lamp are subjected to an image processing so as to be measured; and the like.
On the other hand, in terms of kinds of optical systems used for alignment adjustment, the alignment systems can be roughly divided into TTL (though-the-lens) system in which the position of photosensitive substrate is measured through a projection optical system; TTR (through-the-reticle) system in which the positional relationship between the reticle and the photosensitive substrate is measured through the projection optical system and the reticle; and off-axis system in which the position of photosensitive substrate is directly measured without the aid of a projection optical system.
The illumination light (alignment light) for illuminating the alignment mark may be either nonexposure light having a wavelength which is outside the photosensitive range of the photosensitive layer on the photosensitive substrate and is different from that of exposure light, or the exposure light itself. Since the projection optical system of the projection exposure apparatus is corrected in terms of aberration with respect to exposure light, it may yield a large amount of aberration, such as chromatic aberration in particular, with respect to nonexposure light. Accordingly, in the case where nonexposure light is used as alignment light, a lens for correcting chromatic aberration is employed therewith.